1. Field of the Invention
This invention relates to the field of fingerprint image sensing, acquisition, or detection, that is, to the detection of a fingerprint's minutiae such as ridge endings and bifurcations. More specifically, this invention relates to a solid state, multi-pixel, capacitance type, fingerprint sensor having a new and unusual packaging construction and arrangement.
2. Description of the Related Art
As is well known, the fingerprint of a human is composed of lines or ridges, which have both branching parts or bifurcations and abrupt endings or end points. These branching parts, bifurcations, endings, and end points are also known as minutiae. Fingerprints are identified by the locations of, and the relationships among, these minutiae.
While fingerprint detection is traditionally used in crime detection, fingerprint detection also finds utility as a personal key or signature; for example, to improve the safety of credit card use, to control access to secure areas, computers and banking system, and as a substitute for house or car keys.
The present invention relates to a capacitance distance/fingerprint sensor, such as is described in the above mentioned related patent application, that is packaged in accordance with this invention in a unique manner. The general use of capacitance type sensors is generally known.
For example, the publication SENSORS AND ACTUATORS, January/February 1989, no. 1/2, at pages 141-153, contains an article entitled INTEGRATED TACTILE IMAGER WITH AN INTRINSIC CONTOUR DETECTION OPTION that was presented at the Fourth International Conference on Solid-State Sensors and Actuators (Transducers '87), Tokyo, Japan, Jun. 2-5, 1987. This article describes an integrated capacitive tactile imaging sensor that comprises a multi-layer construction having a bottom ceramic support, a 9-row/9-column array of square aluminum electrodes that are contained on a silicon wafer integrated circuit, a flexible and isolating intermediate layer that is made up of natural rubber, a thin conductive rubber layer, and a top protective layer. In this device, capacitance depends upon local deformation of the natural rubber layer. The 81 individual aluminum electrodes of this device provide capacitive measurement of an indentation pattern within the natural rubber layer, this indentation being caused by a pressure distribution that acts on the top protective layer.
The use of a capacitance type sensor to sense the minutiae of a fingerprint is also known.
For example, the publication IEEE ELECTRON DEVICE LETTERS, VOL. 18, NO. 1, JANUARY 1997, pages 19-20, contains an article entitled NOVEL FINGERPRINT SCANNING ARRAYS USING POLYSILICON TFT'S OF GLASS AND POLYMER SUBSTRATES. This article describes a two-dimensional (2-D), 200.times.200, capacitance sensing array that is made up of 40,000 individual pixels. Each pixel of the array includes two Thin Film Transistors (TFTs) and a capacitor plate. Each array pixel resides at the intersection of an array-row and an array-column, and each array pixel is individually addressable by way of row-driver circuits and column-driver circuits.
Considering the two TFTs, hereinafter called TFT-A and TFT-B, that are associated with a given pixel, the drain electrodes of TFT-A and TFT-B are connected to the pixel's capacitor plate, the gate electrode and the source electrode of TFT-A are connected to a row-conductor that is associated with the pixel, the gate of TFT-B is connected to the following row-conductor, and the source of TFT-B is connected to a column-conductor that is associated with the pixel.
A thin (0.1 micrometer) silicon nitride insulator overlies the capacitor plate of each array pixel. When the ridge of a fingerprint lies directly over the capacitor plate, a capacitor is formed between the capacitor plate and the finger. This capacitor is charged when a row-pulse (8 to 10 VDC, and of 10 to 100 micro second duration) is applied to the pixel by way of the row-conductor that is associated with this pixel and TFT-A. This stored charge is thereafter transferred onto the pixel's column-conductor through TFT-B when a row-pulse is applied to the following row-electrode.
Also of interest is the publication 1997 IEEE INTERNATIONAL SOLID-STATE CIRCUITS CONFERENCE that contains and article beginning page 200 entitled A 390DPI LIVE FINGERPRINT IMAGER BASED ON FEEDBACK CAPACITIVE SENSING SCHEME. This article describes a single-chip, 200.times.200 element array, 2-metal digital CMOS technology, sensor that is based upon feedback capacitance sensing, and that operates to detect the electrical field variation that is induced by the finger's skin surface. In each element of the array two horizontally spaced metal plates are separated from the overlying and adjacent portion of the finger's skin surface by passivation oxide. Since the distance between the skin and the sensor's surface identifies the presence of the fingerprint's ridges and valleys, an array of elements provides a complete fingerprint pattern.
In each element of the array, the two metal plates are respectively connected to the input and the output of a high-gain inverter, to thereby form a charge-integrator. In operation, the charge-integrator is first reset by shorting the input and output of the inverter. A fixed amount of charge is then sinked from the input, causing the output voltage to swing inversely proportional to a feedback capacitance value that is inversely proportional to the distance to the fingerprint's ridges and valleys. The array of cells or sensors thus provides the complete fingerprint pattern. The fingerprint image disappears when the finger is removed from the array.
U.S. Pat. No. 4,353,056, incorporated herein by reference, is of interest in that it relates to a capacitance-type fingerprint sensor wherein a finger is pressed onto the sensor's surface in order to read the ridges and valleys of the fingerprint. The sensor-surface has a large number of capacitors of a small physical size associated therewith. Two sensors are described. In a first type of sensor, an electrical insulator carries a number of flexible and horizontally spaced curved metal electrodes, two adjacent metal electrodes of which comprise one capacitor. A protective insulating film overlies the electrical insulator, and when a finger is brought into physical contact with this protective insulating film, the metal electrodes are physically deformed, thereby selectively changing the capacitance of the large number of capacitors in accordance with the fingerprint's ridge/valley pattern. In a second type of sensor, the top surface of a rigid support carries a number of horizontally spaced and flat metal electrodes in a fixed position. Placed above the plane of the metal electrodes is the sequential arrangement of a flexible insulator, a flexible electrode, and a flexible protective membrane. A capacitor is formed between the top flexible electrode and each of the lower and fixed-position flat metal electrodes. When the end of a finger is brought into contact with the flexible membrane, the flexible electrode becomes wavy in accordance with the fingerprints' ridges/valleys pattern.
In addition, U.S. Pat. No. 5,325,442, incorporated herein by reference, relates to a capacitance-type fingerprint sensor having a sensing pad that comprises a planar array of row/column sensing elements having a pitch of about 100 micrometers. Each sensing element is located at the intersection of a row conductor and a column conductor, and in each sensing element, a sensing capacitor is made up of a planar sensing electrode that is spaced from a finger surface by way of an insulating film that overlies the sensing electrode. The plurality of sensing electrodes that make up the array are regularly spaced and re equal size rectangles.
The sensing elements are fabricated using photolithographic processes, and each individual sensing element includes a Thin-Film-Transistor (TFT) in the form of a Field-Effect-Transistor (FET). Each FET gate is connected to a row conductor, each FET source is connected to a column conductor, and each FET drain is connected to a sensing electrode.
In one embodiment, each sensing element comprises a sensing capacitor that is formed between a sensing electrode and the finger. In another embodiment, each sensing element includes an electrically isolated and conducting pad that is physically engaged by the finger.
While prior devices as above described are generally useful for their limited intended use, a need remains in the art for a solid state capacitance type fingerprint sensing array, wherein the sensor array is assembled or packaged, generally using solid state processing techniques, in a manner that enhances the operation of the sensor array, and at the same time prevents or minimizes contamination and the like when a finger is brought into physical contact with the active upper surface of the sensor array.